HYDROGEOLOGICAL ASSESSMENT OF COMPLEX VOLCANIC SYSTEMS IN CONTINENTAL RIFTED ZONE: UNDERSTANDING GROUNDWATER TABLE AND LAKE LEVEL CHANGES IN MIDDLE AWASH BASIN, ETHIOPIA

Middle Awash basin is one of the complex volcanic environments located in central Main Ethiopian rift. Groundwater flow in this basin converges towards the active tectonic belt of the rift floor constrained by rift faults and massive volcanic hills. Recently, following the construction of large-scale irrigation projects, a notable groundwater table and lake level change has been observed which is related to both natural and man-made factors.

A multidisciplinary approach including geophysical, geochemical, hydrodynamic, numerical groundwater flow modeling and isotopic studies have been applied to characterize the hydrogeological system of the basin to infer cause for groundwater and lake level changes. The results aid in understanding the hydrogeological properties of the rocks and hydrological response of the basin to anthropogenic activities. Hydrochemical, environmental isotope and river hydrograph data are in good agreement indicating the direct consequential effect of precipitation and surface waters on groundwater storage. Furthermore, hydrochemical and isotopic data reveals strong rock-water interaction within the rift driven by a high geothermal gradient in contrast to groundwaters circulating outside of this active tectonic belt. These waters reflect ongoing crustal thinning and propagation of fissures in rift floor that have two important roles: upward circulation of heat and CO₂ from the mantle to the groundwater regime and downward circulation of voluminous surface waters to groundwater. Additionally, 2D tomography and numerical groundwater flow modeling indicates a strong hydraulic link between groundwater and surface waters, and in places, hydraulic head in aquifers are above the bottom elevation of rivers and lake.

Evidence from the various datasets used in this study dictate that the basin is a highly permeable terrain whereby groundwater is highly susceptible to anthropogenic activities such as irrigation projects. Thus, working towards ensuring sustainable development to minimize hydrological perturbation is warranted. Sustainable development will be achieved through full understanding of the hydrogeological behavior of the basin and this study provides basic information towards this effort.